What's the easiest/cheapest variable-frequency sine wave oscillator?

A Google search will give you a few billion ideas. Which is the simplest/easiest/cheapest that you know of?

Generating a square wave and then filtering out the harmonics isn't a good solution unless the filter frequency can be varied along with the square.

Answer

Making a numerically controlled oscillator (NCO) with uC + DAC is very easy. Could be a fun FPGA project. An advantage to an NCO is that you change waveforms.

I did a low frequency numerically controller oscillator Arduino sketch (see http://wiblocks.com/docs/app-notes/nb1a-nco.html). At the bottom of the webpage are a couple of references to the original articles,